For many years, electromagnetic signal pickups have been utilized on musical instruments having ferromagnetic strings. Such pickups have been employed with guitars, bass guitars, banjos, mandolins, and a variety of other instruments. A pickup for a musical instrument that uses ferromagnetic strings almost invariably incorporates a magnetic structure for generating a magnetic field that encompasses the strings. That magnetic structure usually includes at least one permanent magnet and at least one high-permeability pole piece. Frequently, the pickup has a separate pole piece or permanent magnet for each string; thus, a guitar pickup may have six pole pieces or six magnets, one for each string. On the other hand, some electromagnetic pickups have a single pole piece that spans a number of strings, often all of the strings of the instrument. The pickup may have an electrical pickup coil for each string, or it may have one electrical pickup coil that generates a composite all-string signal. The electrical signals from the coil or coils are amplified and reproduced by a speaker or other transducer as the output of the musical instrument. The electrical pickup coils are customarily disposed in encompassing relation to the magnetic cores; when there are plural coils each coil usually has its own core. This relatively simple electromagnetic structure is fitted into a housing. The housing may or may not be part of the magnetic structure. Whether or not it is a part of the magnetic structure, a principal purpose of the housing is to protect the pickup from dirt and other contaminants.
A wide variety of individual constructions have been used for electromagnetic pickups employed with musical instruments such as guitars. Frequently, the efforts of the pickup designer have been directed toward achieving an output signal from the electrical coil that is as close as possible to a faithful reproduction of the sound that would be developed by the instrument functioning as an acoustical device. This is not always the case, however; many electromagnetic pickups have been designed to give a particular distortion deemed desirable by the designer or by a musician.
For electromagnetic pickups in general, as applied to musical instruments having steel or other ferromagnetic strings, there may be some difficulty in obtaining an output signal of sufficient amplitude. This may be a minor problem, with modern electronic technology, because even a very weak signal can often be amplified to an adequate amplitude. On the other hand, a reasonable output amplitude from the pickup itself is desirable because it reduces the necessity for subsequent amplification, and thus reduces the likelihood of inadequately controlled distortion. Moreover, with adequate initial amplitude of the signal generated by the pickup, the signal-to-noise ratio is increased so that a "purer" signal can be realized.
A pronounced problem, in many electromagnetic pickups for musical instruments, has to do with the frequency response. The overall "sound" derived from the output signal is usually critical to the requirements of the musician. Some musicians want to have the output signal as close as possible to the acoustic output of the instrument, at least in theory. Others, however, want to have a distortion that is acceptable to them, one that represents their own concept or technique for interpretation of music. The frequency response characteristics of the pickup are critical in this regard. A similar situation is presented by the sound characteristic known to musicians as "sustain"; sometimes accented "sustain" is desirable in the view of the musician using the pickup and sometimes it is not.